Streptococcus pneumoniae infections are a major cause of human diseases such as otitis media, bacteremia, meningitis, septic arthritis and fatal pneumonia worldwide (Butler et al., 1999; James and Thomas, 2000). Over the past 10-20 years, Streptococcus pneumoniae has developed resistance to most antibiotics used for its treatment. In fact, it is common for Streptococcus pneumoniae to become resistant to more than one class of antibiotic, e.g., β-lactams, macrolides, lincosamides, trimethoprim-sulfamethoxazole, tetracyclines (Tauber, 2000), meaning Streptococcus pneumoniae treatment is becoming more difficult.
Thus, the rapid emergence of multi-drug resistant pneumococcal strains throughout the world has led to increased emphasis on prevention of pneumococcal infections by immunization (Goldstein and Garau, 1997). The currently available 23-valent pneumococcal capsular polysaccharide vaccine, is not effective in children of less than 2 years of age or in immunocompromised patients, two of the major populations at risk from pneumococcal infection (Douglas et al., 1983). A 7-valent pneumococcal polysaccharide-protein conjugate vaccine, recently licensed in the United States, was shown to be highly effective in infants and children against systemic pneumococcal disease caused by the vaccine serotypes and against cross-reactive capsular serotypes (Shinefield and Black, 2000). The seven capsular types cover greater than 80% of the invasive disease isolates in children in the United States, but only 57-60% of disease isolates in other areas of the world (Hausdorff et al., 2000). There is therefore an immediate need for a cost-effective vaccine to cover most or all of the disease causing serotypes of pneumococci. While this can be achieved by adding conjugates covering additional serotypes, efforts continue to find non-capsular vaccine antigens that are conserved among all pneumococcal serotypes and effective against pneumococcal disease.
Protein antigens of Streptococcus pneumoniae have been evaluated for protective efficacy in animal models of pneumococcal infection. Some of the most commonly studied candidate antigens include the PspA proteins, PsaA lipoprotein, and the CbpA protein. Numerous studies have shown that PspA protein is a virulence factor (Crain et al., 1990; McDaniel et al., 1984) but it is antigenically variable among pneumococcal strains. A recent study has indicated that some antigenically conserved regions of a recombinant PspA variant may elicit cross-reactive antibodies in human adults (Nabors et al., 2000). PsaA, a 37 kD lipoprotein with similarity to other gram-positive adhesins, is involved in Mn+ transport in pneumococci (Sampson et al., 1994; Dintilhac et al., 1997) and has also been shown to be protective in mouse models of systemic disease (Talkington et al., 1996). The surface exposed choline binding protein CbpA is antigenically conserved and protective in mouse models of pneumococcal disease (Rosenow et al., 1997). Since nasopharyngeal colonization is a prerequisite for otic disease, intranasal immunization of mice with pneumococcal proteins and appropriate mucosal adjuvants has been used to enhance the mucosal antibody response and thus, the effectiveness of candidate antigens (Yamamoto et al., 1998; Briles et al., 2000).
While the PspA protein, PsaA lipoprotein and the CbpA protein antigens appear promising, it is possible that no one protein antigen will be effective against all Streptococcus pneumoniae serotypes. Laboratories therefore continue to search for additional candidates that are antigenically conserved and elicit antibodies that reduce colonization (important for otitis media), are protective against systemic disease, or both. Thus, there is an immediate need for a cost-effective vaccine to cover most or all of the disease causing serotypes of Streptococcus pneumoniae and methods of diagnosing Streptococcus pneumoniae infection. A better understanding of the genetic and molecular levels of Streptococcus pneumoniae infection will provide the basis for further development of preventative treatments, therapeutic treatments, new diagnostics and vaccine strategies which are specific for Streptococcus pneumoniae. 